1. Field of the Invention
The present invention relates to a carrier for the two-component developer composing of a toner and a carrier, which is used for developing an electrostatic latest image.
2. Description of the Prior Art
The two-component developer composed of a toner and a carrier has long been used for development of the electrostatic latent image formed on the surface of a photosensitive member. The carrier for magnetic brush development is magnetic powder which is available in varied forms. Typical ones are a resin-coated carrier (in which the surface of the carrier is coated with a resin) and a resin-bonded carrier (consisting of a resin and a particulate magnetic substance such as magnetite particles dispersed therein).
The recent magnetic carrier includes iron powder and ferrite powder (see U.S. Pat. No. 3,996,392). The latter has characteristic properties because ferrite is a sintered body of an oxide of trivalent iron and an oxide of a monovalent or divalent metal (such as barium, nickel, zinc, and copper) which varies in properties depending on composition and sintering conditions.
Being a sintered body of oxides, ferrite has a high resistance. The ferrite carrier used for development of electrostatic latent images usually has a resistance in the range of 10.sup.5 to 10.sup.14 .OMEGA.-cm. This resistance permits the toner (contact with the carrier) to acquire as much charge as 20-30 .mu.c/g (in absolute value). A disadvantage of ferrite carrier is that it decreases or increases in resistance as the operating conditions change. Decrease in resistance results in the carrier sticking to the surface of the photosensitive member. Increase in resistance results in the image density decreasing. Moreover, the amount of charge the toner acquires varies depending on the operating conditions. If it increases, the sticking of carrier takes place; if it decreases, the image density increases. In other words, the image density varies depending on the operating conditions.
The recent development in electrophotography includes the back exposure method which, unlike the conventional complicated system using corona discharge, utilizes an electrophotographic system consisting of a glass transparent latent image support, a transparent electrode, a photosensitive member, and a light projecting head. It also includes the cleanerless image forming method which is designed to remove residual toner completely after image transfer so that the subsequent image is not affected. (This method is applied to laser printers and copying machines.) These new systems also use the two-component developer. Unfortunately, the existing one does not meet their requirements. The back exposure method needs a developer to inject as much charge into the photosensitive member as required for development. The high-speed copying machine needs a highly conductive carrier so that the photosensitive member is charged rapidly.
The present inventors' investigation revealed that the foregoing problem is solved if provisions are made so that the carrier has a resistance in the range of 10.sup.2 -10.sup.5 .OMEGA.-cm (which is lower than that of the conventional one) and the toner (in the form of developer) has a charge amount in the range of 5 to 20 .mu.c/g (in absolute value). In fact, it was found that it is difficult to reduce the resistance of ferrite carrier below 10.sup.5 .OMEGA.-cm. The same is true of iron powder carrier. To cope with this situation, there has been proposed a carrier coated with a resin containing electrically conductive fine powder dispersed therein. This carrier has a controlled electrical resistance according to the amount of the electrically conductive fine powder. However, if the electrically conductive fine powder is carbon black, it is necessary to use it in a considerably large amount so as to reduce the resistance as required. To address this problem, there has been proposed a carrier coated with a resin and carbon black consecutively. (See Japanese Patent Laid-open No. 210365/1990.) This carrier has a reduced resistance with a less amount of carbon black. However, it suffers a disadvantage that carbon black is liable to fall from the carrier to foul the photosensitive member.
The conventional resin-bonded carrier has a high volume resistivity because the resin itself is electrically insulating. This produces undesirable effects (such as slow charging) when it is used for a high-speed machine (with the photosensitive drum rotating at a peripheral speed Vp faster than 150-200 mm/s). The result is uneven density in a solid image. Moreover, it is also subject to the operating conditions and hence it causes background fogging at a low temperature and humidity (say, 10.degree. C. and 20% RH) and it gives a low image density at high temperature and humidity (say, 30.degree. C. and 80% RH).
There has recently been proposed an electrically conductive carrier suitable for the developer to be used for the back exposure system. It is characterized by a resin coating on which is formed an electrically conductive layer of carbon black with a controlled average particle diameter. (See Japanese Patent Laid-open Nos. 150538/1993 and 150558/1993.) Controlling the average particle diameter of carbon black (or electrically conductive fine powder) relative to the average particle diameter of carrier is to prevent carbon black from falling from the carrier and to secure a uniform conductive passage on the surface of the carrier. However, the object of imparting conductivity to the carrier is not achieved as expected for carbon black of the same average particle diameter if the combination of a carrier with a toner is not adequate.